Method and apparatus for applying solder to an element on a substrate

ABSTRACT

There is provided a method for applying solder to an element on a surface of a substrate. The method comprises the steps of (a) placing a mold over the surface, where the mold includes a conduit that contains the solder, and (b) heating the solder to a molten state so that the solder flows from the conduit onto the element. The conduit enjoys two degrees of horizontal freedom with respect to the surface such that the conduit becomes substantially aligned with the element when the solder is in the molten state. There is also provided a system for applying solder to an element on a surface of a substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to soldering, and more particularly, toapplying solder to an element on a substrate. The technique isparticularly suitable for applying solder columns to bottom surfacemetallurgy (BSM) pads on a chip carrier.

2. Description of the Prior Art

Electronic packaging generally contains many levels of packages andinterconnections. A first level package may connect one or more siliconchips on a ceramic substrate carrier. A second level package mayinterconnect one or more such ceramic substrate carriers on an organicboard.

The ceramic substrate is connected to the organic board by pins that aretypically rigid and made of metal. The rigid pins are brazed on theceramic substrate with a suitable braze material such as a gold-tinalloy. Ceramic substrates with an array of such pins, i.e., pin gridarrays (PGA), are subsequently plugged into a pin connector or wavesoldered to an array of plated through-holes on the organic board. Thisconnection system has disadvantages such as the through-holes limitingthe number of wiring channels available in the board. Anotherdisadvantage is the high cost associated with the braze material, therigid metal pins, and the pin connectors or plated through-holes.

U.S. Pat. No. 4,914,814 to Behun et al. describes how thesedisadvantages can be avoided by using solder column connection (SCC)technology, which is also known as ceramic column grid array (CCGA)technology. Generally, CCGA technology is less expensive than PGAtechnology. CCGA technology also provides an improved electricalinterconnection that can better withstand stresses associated withthermal expansion mismatch between a ceramic chip carrier and asupporting circuit board.

To connect a ceramic chip carrier to a supporting circuit board usingCCGA technology, the chip carrier is soldered to the board using soldercolumns, which are typically 90% lead and 10% tin. The solder columnsare formed and one end is attached to metallized pads on a surface ofthe ceramic chip carrier. Such pads are provided by a technique known asbottom surface metallurgy (BSM). Then the other end of the soldercolumns, opposite to the ceramic chip carrier, is attached to thecircuit board.

One problem associated with the CCGA assembly process at the modulelevel occurs when the solder columns do not properly join to themetallized pads of the chip carrier BSM surface. The problem occurs whenthere is a misalignment between the solder columns and the BSM pads.Pitches of 1.27 mm and 1.00 mm between BSM pads, center to center, areconventionally available. For the 1.00 mm pitch, the BSM pads have adiameter of about 0.8 mm with a spacing of about 0.2 mm between adjacentcolumns. For the 1.27 mm pitch, the BSM pads have a diameter of about0.86 mm with a spacing of about 0.41 mm between adjacent BSM pads. Assuch, the aforementioned problem is more pronounced for the 1.00 mmpitch, but it is also apparent with the 1.27 mm pitch. Defects due tomisalignment result in a lower product yield, a loss of material and anincreased cost due to rework of the CCGA assembly. Another problem isthe formation of excess solder, i.e., solder “blobs”, on the chipcarrier's BSM surface due to upward force from molten solder during asolder reflow operation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodfor applying solder to an element on a substrate, such as a BSM pad on achip carrier.

It is another object of the present invention to provide such a methodthat reduces possibility of an excess solder “blob” forming between theelements.

It is a further object of the present invention to provide such a methodthat utilizes a solder surface tension effect to self-center the solderon the element.

These and other objects of the present invention are achieved by a firstmethod for applying solder to an element on a surface of a substrate,comprising (a) placing a mold over the surface, where the mold includesa conduit that contains the solder, and (b) heating the solder to amolten state so that the solder flows from the conduit onto the element.The conduit enjoys two degrees of horizontal freedom with respect to thesurface such that the conduit becomes substantially aligned with theelement when the solder is in the molten state.

A second method for applying a solder column to an element on a surfaceof a chip carrier, comprises (a) securing the chip carrier in a fixture,(b) applying a solution having a first flux concentration onto theelement, (c) positioning a mold in the fixture over the chip carriersuch that a conduit in the mold is in a preliminary alignment with theelement, where the conduit contains the solder column, (d) applying asolution having a second flux concentration onto a end of the soldercolumn remote from the element, where the first flux concentration isgreater than the second flux concentration, and (e) heating the soldercolumn to a molten state so that the solder column flows from theconduit onto the element. The conduit enjoys two degrees of horizontalfreedom with respect to the chip carrier such that the conduit becomessubstantially aligned with the element when the solder column is in themolten state.

A first embodiment of the present invention is an apparatus for applyingsolder to an element on a surface of a substrate. The apparatuscomprises (a) a base for holding the substrate, and (b) a mold thatincludes a conduit for containing the solder, where the mold is placedon the base over the surface. The conduit enjoys two degrees ofhorizontal freedom with respect to the surface such that the conduitbecomes substantially aligned with the element when the solder is in amolten state.

A second embodiment of the present invention is an apparatus forapplying a solder column to an element on a surface of a chip carrier,comprising (a) a base for securing the chip carrier, (b) a mold forpositioning on the base over the chip carrier, where the mold includes aconduit that contains the solder, and (c) a weight for placement on themold to limit vertical freedom of the mold with respect to the surface.The conduit enjoys two degrees of horizontal freedom with respect to thechip carrier such that the conduit becomes substantially aligned withthe element when the solder column is in a molten state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a fixture, and a pictorial representationof a method, for aligning solder columns with elements on a substrate,in accordance with the present invention.

FIG. 2 is a flowchart of a method for applying solder to an element on asurface of a substrate, in accordance with the present invention.

DESCRIPTION OF THE INVENTION

The present invention provides for a fixture and method for aligningsolder columns with elements on a substrate. The fixture and method aresuited for aligning solder columns with bottom surface metallurgy (BSM)input/output (I/O) pads on a chip carrier.

The present invention increases production yield when attaching thesolder columns onto the chip carrier. Chip carriers are used as aninterconnection between a module and a card or printed circuit board.The fixture allows a mold that contains the solder columns to movefreely horizontally along X and Y axes with respect to the chip carrierduring solder reflow, and provides a weighted element to restrainundesired vertical movement along the Z axis due to solder surfacetension upward forces. More particularly, the invention utilizes thesolder surface tension effect to self-center the solder columns withrespect to the chip carrier BSM pads along the X and Y axis, and appliesa weight for mass balance in the Z axis. This technique significantlyreduces cast column rework that would otherwise occur due tomisalignment.

FIG. 1 is an illustration of a fixture, generally indicated by referencenumeral 100, for aligning solder columns with elements on a substrate,in accordance with the present invention. Fixture 100 includes a base105, an alignment plate 110, a mold 115 and a weight 120.

FIG. 1 also shows a substrate, e.g., chip carrier 125, with a pluralityof elements, e.g. BSM pads 130, configured in an array on its topsurface. BSM pads 130 are metallic contacts, typically plated with goldand nickel by a metallurgy process. The present invention is describedherein in the context of attaching solder columns to a BSM pads 130,however, the solder is not restricted in form to that of a column, andany suitable conductor, such as a wire, or component, such as anintegrated circuit, can be soldered to BSM pads 130.

Base 105 has a cavity 135 into which chip carrier 125 is placed. Cavity135 allows chip carrier 125 to enjoy two degrees of horizontal freedom,that is, along an X axis and along a Y axis, over a limited spatialrange. Base 105 also includes alignment pins 140. Preferably base 105 ismade of graphite, and alignment pins 140 are made of stainless steel.

Mold 115 includes a plurality of channels or conduits 145 that containthe solder that will eventually be applied to the BSM pads 130. Conduits145 are configured in an array such that each of conduits 145 correspondwith, and ultimately will align with, one of the plurality of BSM pads130. The solder is preferably a mix of 90% lead and 10% tin. It can beinstalled into conduits 145 by any conventional process, however thepreferred installation techniques are either of an injection moldingprocess or a mechanical vibration and vacuum process. In the injectionmolding process, injection molded solder (IMS) is injected, while in amolten state, into conduits 145. In the mechanical vibration and vacuumprocess, also known as a pin load, pre-cut solder segments or solderwires are loaded into conduits 145.

Mold 115 is placed over or on top of chip carrier 125. It has alignmentholes 150 that mate with alignment pins 140 of base 105 and help toensure the alignment of conduits 145 and BSM pads 130. Alignment holes150 have a diameter that is approximately 0.014 to 0.018 inch largerthan the diameter of alignment pins 140. Thus, mold 115, andaccordingly, channels 145, is allowed a limited degree of horizontalfreedom along the X and Y axes. This freedom also allows molten solderfrom conduits 145 to move freely in the X and Y axes with respect to BSMpads 130 during furnace reflow of the solder. Mold 115 is preferablymade of graphite

Weight 120 has alignment holes 155 that mate with alignment pins 140 ofbase 105. It is placed on top of mold 115 and applies a downward forceon mold 115 and restrains vertical movement of mold 115, i.e., along theZ axis.

Alignment plate 110 is a template that is temporarily placed on top ofchip carrier 125 when chip carrier 125 is in cavity 135, to facilitate apreliminary alignment of BSM pads 130 of chip carrier 125 with conduits145 of mold 115. Alignment plate 110 has several apertures 165 thatallow a user of fixture 100 to visually inspect the orientation of chipcarrier 125 within cavity 135. The spacing between the centerlines ofapertures 165 relative to alignment holes 160 is the same as the spacingbetween the centerlines of corresponding conduits 145 relative toalignment holes 150.

During the preliminary alignment, the user places one end of apertures165 adjacent to BSM pads 130, and views BSM pads 130 through the otherend of apertures 165. The user typically performs the visual inspectionwith the aid of a microscope or other suitable magnifying device.Apertures 165 are slightly larger than BSM pads 130, and preferablyslope out to have a larger dimension on the side through which the useris looking than on the side adjacent to BSM pads 130. For example, asshown in FIG. 1, apertures 165 may have side walls that slope out at anangle of about 45 degrees from a vertical axis. The sloped side wallspermit for easier viewing of the relationship between BSM pads 130 andthe end of apertures 165 adjacent thereto. The user performs thepreliminary alignment by adjusting the position of chip carrier 130within cavity 135 for a best fit between apertures 165 and BSM pads 130.After completion of the preliminary alignment, alignment plate 110 isremoved from base 105, and chip carrier 125 is secured in itspre-aligned position.

FIG. 2 is a flowchart of a method 200 for applying solder to an elementon a surface of a substrate, in accordance with the present invention.Method 200 is described below with reference to the elements of FIG. 1,in the context of aligning solder columns with BSM pads. Method beginswith step 205.

In step 205, chip carrier 125 is placed within cavity 135 of base 105.Method 200 then progresses to step 210.

In step 210, alignment plate 110 is placed on top of chip carrier 125and chip carrier 125 is preliminarily aligned within cavity 135. Thepreliminary alignment more specifically refers to a desired alignmentrelationship between BSM pads 130 and conduits 145 when mold 115 isplaced over chip carrier 125 (see step 220, below). Alignment plate 110has several apertures 165 to permit a visual inspection of therelationship between apertures 165 of alignment plate 110 and BSM pads130. Upon completion of this preliminary alignment, alignment plate 110is removed and chip carrier 125 is secured in its pre-aligned positionwithin base 105, by way of a set screw or other conventional lockingarrangement. If the solder surface tension self-centering effect issufficient to yield a satisfactory alignment between conduits 145 andBSM pads 130, then step 210 is not required. Method 200 then progressesto step 215.

In step 215, a flux solution is applied to the surface of BSM pads 130to remove oxides therefrom. The flux solution preferably has a fluxconcentration of about 8% to 12% rosin-based in an organic solvent, suchas iso-propyl-alcohol (IPA). Method 200 then progresses to step 220.

In step 220, mold 115, which contains the solder columns in conduits145, is placed over chip carrier 125. Alignment holes 150 mate withalignment pins 140. Because of the preliminary alignment performedduring step 210, the solder columns are substantially aligned with BSMpads 130. Method 200 then progresses to step 225.

In step 225, a flux solution is applied to the top surface of mold 115,and more specifically, to the end of the solder columns opposite of, orremote from, BSM pads 130, to create a localized reducing atmosphere.The flux solution preferably has a flux concentration of about 2% to 6%rosin-based in an organic solvent, such as IPA. Note that the fluxconcentration on the surface of BSM pads 130 (see step 215 ) is greaterthan that applied to the surface of mold 115 (in the current step)because solder flows to an area of higher flux concentration, and inthis case, the solder is intended to flow toward BSM pads 130. Aftercompletion of step 225, method 200 progresses to step 230.

In step 230, weight 120 is placed on top of mold 115. Weight 120counteracts an upward force from molten solder that ordinarily occursduring a solder reflow operation. It reduces the possibility that soldercolumn will join together as “blobs” during reflow. Weight 120 isselected as an optimized mass balance along the Z axis. If weight 120 istoo heavy, then it will unduly restrict the movement of mold 115 alongthe X and Y axes, and consequently, when the solder is in its moltenstate, the self-centering effect of the solder surface tension will beineffective. If weight 120 is too light, then it will not adequatelyrestrain the aforementioned vertical force to prevent solder “blobs”.Table 1 lists several different chip carrier configurations, and weightsthat are considered practical for those configurations.

TABLE 1 BSM Pad BSM Pad Weight (grams) Weight (grams) Pitch (mm) CountPin Load Process IMS Process 1.27 624 15 to 20 16 to 22 1.27 831 50 to58 55 to 64 1.27 1088 62 to 70 68 to 77 1.00 1247 66 to 74 72 to 81 1.001657 88 to 94  96 to 103

Note that the optimized weights for weight 120 depend, in part, onwhether the solder columns were installed into conduits 145 of mold 115by the pin load process or by the IMS process. For example, given a BSMpad pitch of 1.00 mm and a BSM pad count of 1657, a weight of 88 to 94grams is appropriate for the pin load process, whereas 96 to 103 gramsis appropriate for the IMS process. This is due, in part, to the factthat in the IMS process the solder is installed in a molten state intoconduits 145, and thus conduits 145 are substantially, completely filledwith solder. In contrast, in the pin load process, the solder isinstalled in a solid state in the form of solder segments or solderwires, and thus conduits 145 typically are not completely filled withsolder. Because of this difference in the quantity of solder in conduits145, a greater weight, approximately 10% on average, is suggested foruse if conduits 145 were loaded using the IMS process as compared tobeing loaded using the pin load process.

Also in step 230, assembly 100 is heated, for example in a furnace, totransform the solder columns in conduits 145 to a molten state. Themolten solder flows from conduits 145 to BSM pads 130, thus resulting inthe formation of a cast solder columns at each of BSM pads 130. Becauseof the surface tension of reflowed solder and because of the loose fitbetween alignment holes 105 of mold 115 and alignment pins 140 of base105, the molten solder from conduits 145 moves with relative freedom inthe X and Y directions, and self-centers the solder column with BSM pads130. Note that if the solder surface tension self-centering effect isadequate to achieve a desired level of column alignment on its own, thenthe preliminary adjustment in step 210 is not required.

It should be understood that various alternatives and modifications canbe devised by those skilled in the art. For example, although thepresent invention is described in the context of applying solder columnsto BSM pads on a chip carrier, it is also suitable for applying solderto other types of surface elements, such as pads on a printed circuitboard or contacts on a surface-mount component. The present invention isintended to embrace all such alternatives, modifications and variancesthat fall within the scope of the appended claims.

What is claimed is:
 1. A method for applying solder to an element on asurface of a substrate, comprising: placing a mold over said surface,wherein said mold includes a conduit that contains said solder; andheating said solder to a molten state so that said solder flows fromsaid conduit onto said element, wherein before the heating, applying asolution having a first flux concentration onto said element; andapplying a solution having a second flux concentration onto an end ofsaid solder remote from said element, wherein said first fluxconcentration is greater than said second flux concentration, whereinsaid conduit enjoys two degrees of horizontal freedom with respect tosaid surface such that said conduit becomes substantially aligned withsaid element when said solder is in said molten state.
 2. The method ofclaim 1, further comprising, before said placing, holding said substratein a fixture, wherein said placing comprises positioning said mold insaid fixture such that said conduit is in a preliminary alignment withsaid element.
 3. The method of claim 2, further comprising, before saidholding, using a template to position said substrate in said fixture tofacilitate said preliminary alignment.
 4. The method of claim 1, furthercomprising, before said heating, placing a weight on said mold to limitvertical freedom of said mold with respect to said surface.
 5. Themethod of claim 1, further comprising, prior to said placing, installingsaid solder into said conduit by injection molding.
 6. The method ofclaim 1, further comprising, prior to said placing, installing saidsolder into said conduit as a pin load.
 7. The method of claim 1,wherein said solder comprises a solder column, wherein said elementcomprises a bottom surface metallurgy (BSM) pad, and wherein saidsubstrate comprises a chip carrier.
 8. A method for applying a soldercolumn to an element on a surface of a chip carrier, comprising:securing said chip carrier in a fixture; applying a solution having afirst flux concentration onto said element; positioning a mold in saidfixture over said chip carrier such that a conduit in said mold is in apreliminary alignment with said element, wherein said conduit containssaid solder column; applying a solution having a second fluxconcentration onto a end of said solder column remote from said element,wherein said first flux concentration is greater than said second fluxconcentration; and heating said solder column to a molten state so thatsaid solder column flows from said conduit onto said element, whereinsaid conduit enjoys two degrees of horizontal freedom with respect tosaid chip carrier such that said conduit becomes substantially alignedwith said element when said solder column is in said molten state. 9.The method of claim 8, further comprising, before said securing, using atemplate to position said chip carrier in said fixture to facilitatesaid preliminary alignment.
 10. The method of claim 8, furthercomprising, before said heating, placing a weight on said mold to limitvertical freedom of said mold with respect to said chip carrier.
 11. Themethod of claim 8, wherein said element is a bottom surface metallurgy(BSM) pad.